.TH std::exponential_distribution 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::exponential_distribution \- std::exponential_distribution

.SH Synopsis
   Defined in header <random>
   template< class RealType = double >  \fI(since C++11)\fP
   class exponential_distribution;

   Produces random non-negative floating-point values \\(\\small x\\)x, distributed
   according to probability density function:

   \\(\\small P(x|\\lambda) = \\lambda e^{-\\lambda x}\\)P(x|λ) = λe-λx

   The value obtained is the time/distance until the next random event if random events
   occur at constant rate \\(\\small\\lambda\\)λ per unit of time/distance. For example,
   this distribution describes the time between the clicks of a Geiger counter or the
   distance between point mutations in a DNA strand.

   This is the continuous counterpart of std::geometric_distribution.

   std::exponential_distribution satisfies RandomNumberDistribution.

.SH Template parameters

   RealType - The result type generated by the generator. The effect is undefined if
              this is not one of float, double, or long double.

.SH Member types

   Member type         Definition
   result_type \fI(C++11)\fP RealType
   param_type \fI(C++11)\fP  the type of the parameter set, see RandomNumberDistribution.

.SH Member functions

   constructor   constructs new distribution
   \fI(C++11)\fP       \fI(public member function)\fP
   reset         resets the internal state of the distribution
   \fI(C++11)\fP       \fI(public member function)\fP
.SH Generation
   operator()    generates the next random number in the distribution
   \fI(C++11)\fP       \fI(public member function)\fP
.SH Characteristics
   lambda        returns the lambda distribution parameter (rate of events)
   \fI(C++11)\fP       \fI(public member function)\fP
   param         gets or sets the distribution parameter object
   \fI(C++11)\fP       \fI(public member function)\fP
   min           returns the minimum potentially generated value
   \fI(C++11)\fP       \fI(public member function)\fP
   max           returns the maximum potentially generated value
   \fI(C++11)\fP       \fI(public member function)\fP

.SH Non-member functions

   operator==
   operator!=                compares two distribution objects
   \fI(C++11)\fP                   \fI(function)\fP
   \fI(C++11)\fP(removed in C++20)
   operator<<                performs stream input and output on pseudo-random number
   operator>>                distribution
   \fI(C++11)\fP                   \fI(function template)\fP

.SH Notes

   Some implementations may occasionally return infinity if RealType is float. This is
   LWG issue 2524.

.SH Example


// Run this code

 #include <iomanip>
 #include <iostream>
 #include <map>
 #include <random>
 #include <string>

 int main()
 {
     std::random_device rd;
     std::mt19937 gen(rd());

     // if particles decay once per second on average,
     // how much time, in seconds, until the next one?
     std::exponential_distribution<> d(1);

     std::map<int, int> hist;
     for (int n = 0; n != 10000; ++n)
         ++hist[2 * d(gen)];

     for (auto const& [x, y] : hist)
         std::cout << std::fixed << std::setprecision(1)
                   << x / 2.0 << '-' << (x + 1) / 2.0 << ' '
                   << std::string(y / 200, '*') << '\\n';
 }

.SH Possible output:

 0.0-0.5 *******************
 0.5-1.0 ***********
 1.0-1.5 *******
 1.5-2.0 ****
 2.0-2.5 **
 2.5-3.0 *
 3.0-3.5
 3.5-4.0

.SH External links

   Weisstein, Eric W. "Exponential Distribution." From MathWorld — A Wolfram Web
   Resource.
